The present invention relates generally to gain control circuitry for radio transmitters, and, more particularly, to a gain control circuit for a radio transmitter operative to transmit modulated signals in discrete bursts.
A communication system is operative to transmit information (referred to hereinbelow as an "information signal") between two or more locations, and includes, at a minimum, a transmitter and a receiver interconnected by a communication channel. A radio communication system is a communication system in which the transmission channel comprises a radio-frequency channel wherein the radio-frequency channel is defined by a range of frequencies of the communication spectrum.
The transmitter, which forms a portion of a radio communication system, includes circuitry for converting the information signal into a form suitable for transmission thereof upon a radio-frequency channel. Such circuitry includes modulation circuitry which performs a process referred to as modulation. In such a process, the information signal which is to be transmitted is impressed upon a radio-frequency electromagnetic wave.
The radio-frequency electromagnetic wave upon which the information signal is impressed is of a frequency within a range of frequencies defining in the radio-frequency channel upon which the information is to be transmitted. The radio-frequency, electromagnetic wave is commonly referred to as a "carrier signal," and the radio-frequency, electromagnetic wave, once modulated by the information signal, is commonly referred to as a modulated signal.
Various modulation schemes are known for impressing the information signal upon the carrier signal to form thereby the modulated signal. For instance, amplitude modulation, frequency modulation, phase modulation, and combinations thereof are all modulation schemes by which an information signal may be impressed upon a carrier wave to form a modulated signal.
Radio communication systems are advantageous in that no physical interconnection is required between the transmitter and the receiver; once the information signal is modulated to form a modulated signal, the modulated signal may be transmitted over large distances.
A two-way, radio communication system is a radio communication system, similar to the radio communication system described above, but which further permits both transmission of information to a location and transmission of information from that location. Each location of such a two-way, radio communication system contains both a transmitter and a receiver. The transmitter and receiver positioned at a single location typically comprise a unit referred to as a radio transceiver, or, more simply, a transceiver.
Conventionally, transceivers constructed to be operated in two-way communication systems are operative to transmit a modulated signal upon a first radio frequency channel and to receive a modulated signal transmitted upon a second, frequency channel. Because signals transmitted to and by such transceivers are transmitted upon different radio frequency channels, simultaneous two-way communication between two or more transceivers is permitted. Signals are continuously transmitted upon each of the two radio frequency channels to effectuate the two-way communication. Such two-way communication is oftentimes referred to generally by the term duplex operation of the transceiver.
Certain frequency bands of the electromagnetic frequency spectrum have been allocated for such two-way communication. For instance, a frequency band extending between 800 MHz and 900 MHz has been allocated in the United States for cellular communication systems. A plurality of radio frequency channels have been defined in such frequency band permitting operation of numerous cellular telephones (which constitute a type of transceiver construction) thereon. Conventionally, transceivers operative in cellular, communication systems generate modulated signals by a frequency modulation technique. Another frequency band has been allocated for communication by cordless telephones (which constitute another type of radio transceiver construction).
Increased popularity of use of such two-way communication systems has resulted in increased demand for access to the finite number of radio frequency channels allocated for such two-way communication. Schemes have been developed to utilize more efficiently the frequency bands allocated for such use.
Several of such schemes involve the modulating in encoded form the information signal. In such a process, the signal becomes compacted, and the signal may be transmitted more efficiently (i.e., the same amount of information may be transmitted in a lesser amount of time). Additionally, a modulated signal formed by such a process need not be transmitted continuously; rather, the modulated signal may be transmitted in discrete bursts.
To optimize the use of the available frequency spectrum, it is often advantageous to modulate both the phase and amplitude of a signal. One such scheme by which an information signal may be encoded is a .pi./4 shifted quarternary phase shift keying (.pi./4-QPSK) modulation scheme. In such a modulation scheme, the information content of a modulated signal formed thereby is contained in the phase of the modulated signal. However, rapid phase changes at the data rates used in these communications systems results in large frequency deviations of the modulated signal which, in turn, results in wide frequency channels. To improve spectrum efficiency, the baseband modulation is frequency limited (filtered). This filtering results in significant amplitude modulation of the modulated signal.
A .pi./4-QPSK-modulated signal may be transmitted in discrete bursts. By transmitting the modulated signal in discrete bursts, transmission of more than one signal at the same frequency is permitted. Such a technique is referred to as time-division multiple access (TDMA).
However, as the bandwidth is controlled by the amplitude of a .pi./4-QPSK-modulated signal, care must be exercised when amplifying such a signal. Amplifier circuitry forms a portion of radio transmitter circuits to amplify a modulated signal prior to transmission thereof.
Gain control circuitry forms a portion of many radio transmitter circuits. Such circuitry controls the signal level of the modulated signal transmitted by the radio transmitter circuit, and typically includes amplifier circuitry for amplifying a modulated signal applied thereto. By controlling the level of amplification of the amplifier circuitry (i.e., by controlling the gain of the amplifier circuitry), the signal level of the amplified, modulated signal transmitted by the radio transmitter may be controlled.
Gain control circuitry may be constructed to determine the average power of a .pi./4-QPSK-modulated signal, and the amount of amplification of such a modulated signal may be controlled responsive to such measurement of average power. However, due to the amplitude modulation component, measurement of the average power requires measurement of the signal level of the modulated signal for a period of time. Such a required time period prior to a determination of the appropriate amount of gain of the modulated signal prevents an accurate determination of desired gain of the modulated signal until the required time period has elapsed and the desired gain has been determined.
When utilizing such conventional gain control circuitry in a radio transmitter operative to transmit an amplitude modulated signal in discrete bursts, the appropriate level of amplification of the amplifier circuitry can not be determined within the time period corresponding to the initial discrete burst in which the modulated signal is transmitted.
Alternatively, while the desired level of gain may be initially estimated, such estimation may be inaccurate as such an estimation of desired gain is based upon estimated, and not actual, parameters.
What is needed, therefore, is a gain control circuit operative to determined quickly the desired amount of gain of a modulated signal.